Recent concerns about increased accumulations of greenhouse gases in the atmosphere have stimulated interest in developing better crop management practices to decrease N2O emissions from agricultural soils. Agriculture is the single largest source of anthropogenic N2O emissions (Bouwman et al., 2005). Currently, agricultural N2O emissions are more than twice that of pre-1940 management practices and about six times more than from native vegetation (Del Grosso et al., 2005). Nitrogen fertilization is considered the major source of agricultural N2O emissions, contributing 60 to 80% of total emissions on a global scale (Dalal et al., 2003; FAO, 2008). To meet growing demands for food, however, N fertilization is needed to optimize crop yields. Thus, considerable effort is being spent extensively studying fertilization practices to reduce N2O emissions.
Estimations of N2O emissions from N fertilizers applied to agricultural crops vary widely because N2O fluxes depend on many factors, such as the type of N fertilizer and the amount of N applied (Eichner, 1990). For instance, losses of N2O are greater with NH4NO3 than with urea (Harrison and Webb, 2001). Also, N2O emission rates are 0.04% for NO3, 0.15 to 0.19% for NH4 and urea, and 5% for anhydrous NH3 (Breitenbeck et al., 1980; Slemr and Seiler, 1984). The concentrations of NH4 and NO3 in the soil, however, have a greater effect on N2O emissions than the specific fertilizer type applied (Harrison and Webb, 2001).
Microbial interactions in the soil are a very important aspect of N2O emissions from agricultural soils. Native soil microorganisms are responsible for the degradation and conversion of different forms of N in the soil. The most important chemical reactions that take place in the N cycle are mineralization, immobilization, nitrification, denitrification, N2 fixation, and volatilization. These chemical reactions are largely affected by environmental conditions such as temperature and soil moisture. Because environmental conditions are constantly changing, the interactions among all the chemical reactions are very dynamic. Harrison and Webb (2001) suggested that denitrification is the main process responsible for N2O emissions under anaerobic soil conditions, while nitrification accounts for emissions under aerobic soil conditions.
Due to the great importance of the soil microbial community in N cycling in the soil, alterations in community composition and abundance can change the rate of N cycle processes (Cavigelli and Robertson, 2000). Hence, manipulating native soil microbial communities by chemical treatments or by inoculation with selected microorganisms can potentially alter N cycling in the soil. For example, adding nitrification inhibitors is a widely used method to reduce the rate of nitrification by inhibiting autotrophic NH3-oxidizing bacteria (Singh and Verma, 2007).
During the past few decades, there has been increased interest in the use of beneficial microbial inoculations to improve plant and soil functions. Several microorganisms, such as plant growth-promoting rhizobacteria (PGPR), have been widely studied (Figueiredo et al., 2010). The PGPR stimulate plant growth through either a “biofertilizing” effect or a biocontrol effect. There is currently much interest in PGPR and other microbial-based inoculants specifically as alternatives to or supplements with fertilizers to improve the uptake of nutrients (Adesemoye et al., 2009, 2010; Canbolat et al., 2006; Idriss et al., 2002). Among the PGPR microorganisms, Bacillus spp. are widely used, mainly because they can survive as spores and can potentially alter the soil microbial composition. Bacillus spp. have a wide metabolic capability that allows them to play important roles in soil ecosystem functions and processes. Due to their heterotrophic nature, Bacillus spp. play an important role in the soil C cycle, soil N cycle, soil S cycle, and transformation of other soil nutrients (Mandic-Mulec and Prosser, 2011). Furthermore, they work as biocontrol agents due to the wide range of antiviral, antibacterial, and antifungal compounds they produce, which can control pathogens and have an effect on other soil microorganisms (Chaabouni et al., 2012). Antibiotics are important metabolites that are produced by Bacillus spp. They not only can control pathogens but also confer a competitive advantage over other soil microorganisms (Stein, 2005).
Although the use of microbial-based inoculants is increasing, currently there is a lack of information about how these products affect N2O emissions from soils when N fertilizers are present.